Considerable interest exists for static dissipative and electrically conductive laminates for use in various environments, including static dissipative work surfaces and conductive flooring materials. Among the prior patents there may be mentioned are the patents to Wilks et al U.S. Pat. No. 3,922,383; Cannady et al U.S. Pat. No. 4,480,001; Cannady U.S. Pat. No. 4,540,624; Berbeco U.S. Pat. No. 4,454,199 and Berbeco U.S. Pat. No. 4,455,350. The use of carbon black filled paper is known, as is the use of salts, noting patents such as Meiser U.S. Pat. No. 3,650,821 and Economy et al U.S. Pat. No. 3,567,689. However, no one product is suitable for all static dissipative and conductive environments, because different usages, i.e. environments, require different properties.
Grosheim et al U.S. Pat. No. 4,472,474 belongs to the prior art mentioned above, but is of further interest in disclosing the use of an electrically conductive fibrous web in order to impart greater conductivity to the laminate, this conductive web being preferably highly loaded with conductive particles. As a non-preferred alternative, conductive fibers can be used. Example 5 specifically mentions the use of carbon fiber-containing conductive webs of 90% and 40% carbon fibers.
Cannady U.S. Pat. No. 4,540,624 discloses anti-static laminates containing long carbon fibers, these carbon fibers being uniformly distributed throughout at least the to decorative sheet of the laminate.
Prior static dissipative laminates suffered from certain disadvantages in addition to being either too conductive or not conductive enough. Thus, some of these static dissipative laminates have an upper surface containing carbon particles for providing a conductive path from the upper surface of the laminate to the interior. This can result in dusting of conductive material from the surface of the laminate as it wears, which conductive material by itself will result in damage due to electrical short circuits. In addition, the color of these laminates is limited to black, which can provide human engineering problems.
Another problem which occurs with such prior static dissipative laminates is that the surface of the laminate tends to lose its electrical conductivity when the relative humidity drops in winter time. Measured resistivity of conventional static dissipative and conductive laminates is strongly dependent on relative humidity, and can change several orders of magnitude between 50% relative humidity and 15% relative humidity. Prior art static dissipative and conductive laminates do not perform well at relative humidities below 25-30%. For this reason, work areas may have to be humidified, which is not always desirable due to the possibility of inducing corrosion in certain products and in certain equipment as well. In addition, the necessity for precise humidification increases the cost of handling the electronic components.
Two of the high pressure decorative laminates having static dissipating or conducting properties use a highly conductive impregnated layer below the decor sheet. Of these, one has an excessive surface resistivity and it appears that the upper layer is not sufficiently conductive. The other uses quarternary ammonium compounds in the upper layer, along with the conductive carbon containing paper therebelow, and while this laminate is adequate at normal relative humidity (about 50%), it is inadequate at low relative humidities. A third product of yet another manufacturer, although somewhat better, is still inadequate at low relative humidities.
Prior art static dissipative laminate has also introduced the problem of field suppression. This occurs when the laminate is constructed of a highly conductive layer buried under a relatively non-conducting surface. When a charged object is placed on the laminate surface, a field is induced in the buried conductive layer forming what is, in effect, a leaky capacitor. The overall result is that to an outside observer, e.g. a static electricity sensing meter such as an electrometer, a zero electrical potential exists when, in reality, the field is hidden within the laminate. When an object such as an electronic component is lifted from the laminate surface, the charge reappears thereby creating the static electricity hazard sought to be avoided.
An excellent static dissipative laminate, described in Ungar et al U.S. Pat. No. 4,784,908, which laminate has served the industry well, uses a carbon particle filled paper two layers down from the surface decorative laminate. One of the advantages of this static dissipative laminate is that it has a zero volt charge after two seconds at 17% or lower relative humidity (column 3, lines 23-26). The carbon black paper used in the laminate exemplified in this patent contains a non-uniform dispersion of carbon particles which, in the finished laminate, tend to enhance the conductivity of that layer. The carbon particles are extremely small, submicron in size. Thus, electrical continuity in the layer is dependent on high concentrations of carbon particles to achieve low resistances. Even then, the resistance is somewhat dependent on resin content because the greater the resin content, the more the resin coats each individual particle and insulates it from neighboring particles.
Moreover, the use of carbon black paper creates tremendous handling and control problems. The carbon black paper has a highly non-uniform distribution of carbon which creates widely varying electrical properties locally within the paper. Also, the wet tensile strength varies throughout a roll of such carbon black paper as a result of the uneven carbon distribution, often making it difficult to uniformly impregnate with resin.
Another disadvantage of the carbon black paper is that the yields using this carbon black paper have not been found to be optimal because of wastage, partly due to some of the aforementioned problems. There have also been found to be cross-contamination problems with the use of carbon black in decorative laminate processes.